A power supply for a dc arc lamp is frequently provided with an automatic restart feature, whereby if the lamp is extinguished (due to, say, supply voltage fluctuations), the supply will sense this condition and automatically restart the lamp. The traditional restart method is based on sensing when the lamp voltage has exceeded a certain threshold value. When this condition is sensed, a lamp igniter circuit is activated until the lamp restarts and the lamp voltage drops below the threshold. Under certain circumstances toward the end of lamp life, however, the operating lamp voltage will rise, and the restart circuit may attempt to restart an operating lamp. This can result in destruction of the restart circuit.
It is also known to use lamp current as the restart criterion for ac lamps. A typical example of current control is shown in U.S. Pat. No. 4,763,044, in which a current sensing relay is connected in series in the lamp return line. If current is interrupted, the relay closes and thereby applies a series of high voltage oscillations to the lamp to restart conduction through the lamp. The relay then opens at some current value short of the operating current, and the lamp continues to draw ac current from the ac supply.
AC supplies, such as the one described in the '044 patent, provide considerable ac ripple in the drive current for an ac lamp. The resulting variations in lamp illumination is unimportant for the usual applications of such lamps, such as for street lamps. However, such ripple cannot be tolerated in systems where the level of illumination is being measured, as, e.g., in a film scanning system in which the lamp illuminates a transparent film and the transmitted illumination is measured by a photosensor. Consequently, the skilled designer of film scanners would not turn to such supplies to power a scanner lamp.
It is accordingly desirable to use a constant current dc lamp in a film scanning system. Ripple is ordinarily not evident and the illumination can be very constant. The need remains, however, to restart such a lamp without endangering the restart circuit as the lamp ages, such as the known voltage restart circuits are prone to do.